As the term is used herein, "transparency sheets" are substrates formed of transparent material, generally in the form of sheets, on which an image can be created for use with equipment which is capable of projecting the image by the transmission of light through the substrate. A "transparency" is generally a transparency sheet on which an image has been printed. When images printed on transparency sheets are used by printing machines, such as photocopiers, laser printers, ink jet printers or thermal transfer printers, with optical sheet detection capability, it is necessary that they have an indicator such as a white stripe affixed to them in order for the sheet to be detected by the machine. Generally, however, this white stripe is opaque, and thus projects black when light is applied to the printed transparency sheet. When this occurs, image information printed on the transparency sheet in the area of the white stripe will not be visible on the projected image, because the portion of the transparency sheet treated with the white stripe will project black. Thus, a transparency sheet which is capable of being detected optically but which also permits all image information on the transparency sheet to be projected is needed.
U.S. Pat. No. 3,852,093 to O'Leary discloses a transparent colored film product, which becomes clear and colorless at image areas when heated thermographically. This film contains a p-quinoneimine color body and a mild reducing agent. The p-quinoneimine color bodies, supplied in a matrix of film-forming binder as a coating on paper, transparent film, etc., undergo decolorization when heated at thermographically induced temperatures in reactive association with a mild reducing agent. Brief heating of the film product causes migration of at least one of the reactants and results in a decolorization of the color body at the heated areas to form an image. The product is said to be useful in making negative color projection transparencies.
U.S. Pat. No. 4,818,742 to Ellis discloses a heat responsive recording element having a recording layer containing a colorless di- or tri- aryl compound having a closed ring moiety incorporating the meso carbon atom and containing a nitrogen atom directly bonded to both the meso carbon and an arylating group that has a displaceable entity. Upon heating the recording layer, the arylating group effects intramolecular arylation of the nitrogen atom with irreversible breaking of the meso carbon atom-nitrogen atom bond, thus rendering the compound colored in the imagewise heating pattern.
U.S Pat. No. 4,837,061 to Smits et al. discloses a laminated tamper-evident structure which exhibits an irreversible color change upon delamination. The patent discloses a structure which includes a laminate of at least two layers capable of generating a color by a light interference and absorption phenomenon that requires direct and intimate contact between adjacent layers. Upon separation of the layers of the laminate, the color generated by the contact between the two layers is irreversibly lost, providing evidence that the structure has been tampered with.
U S Pat. No. 4,032,687 to Hornsby, Jr. discloses an applique, attachable by a pressure sensitive adhesive, which includes a base sheet, a layer of color changeable liquid crystalline material disposed upon the base sheet, and a transparent covering layer overlying the liquid crystalline layer. The applique is color changeable by the application of heat to the liquid crystalline layer.